The present invention relates to a stencil for use in a stencil printer, and a drum and an ink holding member for the stencil printer.
A digital stencil printer using a thermosensitive stencil i s extensively used as a simple and convenient printer. Usually, the thermosensitive stencil has a laminate structure comprising an about 40 .mu.m to 50 .mu.m thick porous substrate and a thermoplastic resin film as thin as about 1 .mu.m to 2 .mu.m and adhered to the substrate. The porous substrate is implemented by Japanese paper fibers, synthetic fibers or a mixture thereof. There has recently been proposed a stencil consisting only of a thermoplastic resin film. The stencil printer includes a rotatable drum having a porous hollow cylinder and an ink holding layer covering the outer periphery of the cylinder. The ink holding layer is constituted by a mesh screen of resin fibers or metal fibers. The thermoplastic resin film has its surface perforated by heat in accordance with image data so as to turn out a master. After the master has been wrapped around the drum, ink is fed to the drum by ink feeding means disposed in the drum. A press roller or similar pressing means continuously presses a sheet against the drum. As a result, the ink oozes out from perforations formed in the master via pores formed in the drum, forming an image on the sheet.
A problem with the stencil printer is that when the printer is operated after being left unaerated over a certain period of time or after a stop of printing, defective printing is apt to occur due to the evaporation of the ink. To solve this problem, it has been customary to use oil ink or water-in-oil emulsion ink which evaporates little. However, because this kind of ink does not dry rapidly, a certain period of time is necessary for the ink to infiltrate into the sheet and dry to such a degree that it does not smear the sheet when rubbed by a finger or the like.
In the stencil printer, sheets carrying images thereon, i.e., printings are sequentially stacked on a tray. If a printing is immediately laid on a printing existing on the tray, the ink on the front of the former deposits on the rear of the latter and smears it. This kind of smearing, or so-called offset, occurs frequently when the amount of ink, particularly the thickness (or height) of ink, forming an image on a sheet is great.
Further, the drum or the master wrapped therearound does not allow the ink to cut or stop sharply. During printing, adhesion acts between the ink on the surface of the master and a sheet when the sheet is separated form the surface of the drum. At this instant, the ink is filled in bores formed in an ink holding member (or porous substrate) and bores formed in a porous support included in the drum. The above adhesion acting between the ink and the sheet causes the ink to be drawn out via perforations formed in the master. As a result, the ink is transferred to the sheet in an excessive amount. It is therefore almost impossible to reduce the amount of the ink to be transferred to the sheet so as to obviate offset.
In light of the above, there has been proposed an ink holding member (or porous substrate) in the form of a mesh screen whose bores are reduced in size. With this configuration, it is possible to reduce the amount of the ink to be drawn out. However, this cannot be done unless the mesh screen (or Japanese paper) constituting the ink holding member (or porous substrate) is #1,000 to #3,000 in terms of mesh size. This kind of mesh screen results in a disproportionately expensive ink holding member (or porous substrate).
Furthermore, the conventional ink holding member or porous substrate includes many portions where the ink flows out straight, i.e., with high fluidity. In such portions, the ink flows out in a great amount and causes offset to occur. In addition, the conventional porous substrate is low in tensile strength, oil resistance and water resistance and apt to extend or break during printing. This limits resistivity to printing available with the stencil.
Conventional technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-258706, 1-204781 (corresponding U.S. Pat. No. 4,911,069), 59-218889, 1-267094, and 57-51485 (corresponding Japanese Patent Publication No. 63-59393), Japanese Utility Model Publication Nos. 5-41026 and 59-229, and European Publication No. 0 127 192.